This invention relates generally to transcritical refrigeration systems and, more particularly, to control systems for transcritical refrigeration systems.
A transcritical refrigeration system or cycle is one where the high side pressure of the refrigerant fluid exceeds the critical pressure of the refrigerant fluid and the low side pressure of the refrigerant fluid is less than the critical pressure of the refrigerant fluid. Transcritical refrigeration systems are increasing in importance. For example, carbon dioxide has received increasing consideration for use as a refrigerant. Some of the advantages provided by carbon dioxide include lower toxicity, zero ozone depletion potential and negligible direct global warming impact. Application of carbon dioxide as a working fluid for automobile air conditioning systems has received considerable commercial attention. In particular, it is anticipated that carbon dioxide will substantially displace the use of R134a in new automobiles over the next 5 to 10 years. Typical heat rejection temperatures for air conditioning systems designed for comfort cooling will exceed the critical temperature of carbon dioxide (87.8xc2x0 F., 1066.3 psia) The rejection of process heat to the environment necessitates that the condenser (or more appropriately the gas cooler) pressure exceed the critical pressure. Since typical evaporation temperatures (40xc2x0 F.) lie below the critical temperature of carbon dioxide the overall cycle is transcritical.
The design and operation of transcritical refrigeration or heat pump cycles pose a unique optimization and control problem. In general, the desired evaporator temperature and/or heat load is known. Typically the ambient utility (water/air) conditions used for heat rejection is also known. In a standard vapor compression cycle, the high side pressure is set by the condition of achieving a saturated or subcooled liquid at the exit of the condenser. In a transcritical cycle, the high side pressure may be selected from a broad range. Unfortunately, only one point of operation will result in minimum power consumption. Given the cited parameters, the objective of any transcritical process control strategy must be to identify the optimal pressure and to drive the process toward it. During actual process operation most systems may deviate substantially from the design load and utility conditions (air-water temperature). In such situations, the power consumption may be 5-10% higher than necessary if the high-side pressure is not adjusted appropriately. Most control systems cannot readily extract this additional process efficiency because they are incapable of adequately determining the optimal high side pressure. Current approaches to this problem rely upon rudimentary techniques such as manual trial and error or complicated heuristics.
Accordingly it is an object of this invention to provide an improved method for operating a transcritical refrigeration system.
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is:
A method for operating a transcritical refrigeration system comprising:
(A) compressing a refrigerant fluid in a compressor to be at a supercritical pressure, passing the compressed refrigerant fluid to a heat exchanger, cooling the compressed refrigerant fluid in the heat exchanger, withdrawing the cooled compressed refrigerant fluid from the heat exchanger, and expanding the resulting refrigerant fluid to a subcritical pressure, said subcritical pressure refrigerant fluid being at least in part in liquid form;
(B) vaporizing subcritical pressure refrigerant fluid to provide refrigeration to a heat load, passing vaporized refrigerant fluid to the heat exchanger, warming the vaporized refrigerant fluid by indirect heat exchange with the cooling compressed refrigerant fluid, withdrawing the resulting warmed refrigerant fluid from the heat exchanger, and passing the withdrawn refrigerant fluid to the compressor;
(C) ascertaining at least two of the two inlet temperatures of the refrigerant fluid passed into the heat exchanger and the two outlet temperatures of the refrigerant fluid withdrawn from the heat exchanger, and ascertaining the enthalpy change of the vaporizing subcritical pressure refrigerant;
(D) monitoring an operating parameter of the compressor, and using the said ascertained temperatures and the said ascertained enthalpy change to determine a more efficient value for said operating parameter; and
(E) adjusting the operation of the compressor so that the value of said operating parameter is closer to the said more efficient value.
Another aspect of the invention is:
A method for operating a transcritical refrigeration system comprising:
(A) compressing a refrigerant fluid in a compressor to be at a supercritical pressure, passing the compressed refrigerant fluid to a heat exchanger, cooling the compressed refrigerant fluid in the heat exchanger, withdrawing the cooled compressed refrigerant fluid from the heat exchanger, and expanding the resulting refrigerant fluid to a subcritical pressure said subcritical pressure refrigerant fluid being at least in part in liquid form;
(B) vaporizing subcritical pressure refrigerant fluid to provide refrigeration to a heat load, passing vaporized refrigerant fluid to the heat exchanger, warming the vaporized refrigerant fluid by indirect heat exchange with the cooling compressed refrigerant fluid, withdrawing the resulting warmed refrigerant fluid from the heat exchanger, and passing the withdrawn refrigerant fluid to the compressor;
(C) ascertaining at least two of the two inlet temperatures of the refrigerant fluid passed into the heat exchanger and the two outlet temperatures of the refrigerant fluid withdrawn from the heat exchanger, and ascertaining the enthalpy change of the vaporizing subcritical pressure refrigerant;
(D) monitoring an operating parameter of the compressor, and using the said ascertained temperatures and the said ascertained enthalpy change to determine a more efficient value for said operating parameter; and
(E) adjusting the working mass of the refrigerant fluid so that the value of said operating parameter is closer to the said more efficient value.
As used herein the term xe2x80x9cworking massxe2x80x9d means the portion of the refrigerant fluid within the compressor, expansion device, process heat exchanger, and associated interconnecting piping of the refrigeration system. Another way of defining the working mass of the refrigerant is as the integrated volume of refrigerant fluid being actively passed through the compressor, i.e. the volume of refrigerant fluid that is passed through the compressor in the time it takes for a refrigerant fluid molecule to make one complete pass through the refrigeration system or refrigeration circuit.
As used herein the term xe2x80x9ccritical pressurexe2x80x9d means the pressure of a fluid at which the liquid and vapor phases can no longer be differentiated.
As used herein the term xe2x80x9ccritical temperaturexe2x80x9d means the temperature of a fluid above which a distinct liquid phase can no longer be formed regardless of pressure.
As used herein the term xe2x80x9centhalpyxe2x80x9d means a thermodynamic measure of heat content per unit mass.